I was happily hoeing a row of potatoes last week when I came upon a couple of plants that were severely wilted. This struck me as curious given the copious amounts of rain that we have received and the vitality and lush growth of the neighbouring potatoes as shown in the following photos.
I checked the plants for any signs of disease, insert damage or physical trauma that might have been caused by my hoe or by marauding mammals. Finding no obvious cause, I spread my search a bit broader and quickly came upon what I believe to be the culprit.
Yes, it is the commonly occurring black walnut (Juglans nigra)! Most members of the Walnut family (Juglandaceae) produce a chemical called “juglone”. Black walnut and butternut trees produce the largest quantity of juglone and can cause toxic reactions with a number of other plant species that grow in their vicinity.
While many plants are tolerant to juglone and grow well in close proximity to walnut trees, there are certain susceptible plant species whose growth can be affected by walnut trees. Through observation and experience, many plant species have been classified as either ‘susceptible’ or ‘tolerant’ to walnut family members.
Experimentally, juglone has been shown to be a respiration inhibitor, which deprives sensitive plants of needed energy to enable metabolic activity. Affected plants cannot exchange carbon dioxide and oxygen properly. In affected plants, xylem vessels become plugged by callus tissue, blocking upward movement of water in the plant.
Symptoms of walnut toxicity range from stunting of growth, to partial or total wilting, to death of the affected plant. The toxic reaction often occurs quickly where sensitive plants can go from healthy to dead within one or two days. I have seen healthy two-foot tall tomato plants collapse overnight. Once wilting begins, the effect cannot be reversed. The severity of the toxic symptoms can vary depending on the plant species that is in contact with the juglone.
Juglone from decomposing black walnut roots can persist in the soil for more than a year after walnut trees have been removed. Don’t assume that roots go out only as far as the drip line. Walnut roots may extend 50 to 80 feet away from the outer canopy of mature walnut trees. It is suggested that young walnut trees may not cause toxic reactions with sensitive plants until the trees are seven to eight years old, but I’m pretty sure that I’m not willing to bet the tomato harvest on this!
Raked up leaves, twigs and husks from walnut trees should be composted for one year to ensure all juglone has broken down prior to spreading into gardens or used as mulch around sensitive plants.
Gardens should be located away from black walnut and butternut trees to prevent damage to susceptible plants. Where close proximity is unavoidable, raised garden beds can provide some protection from juglone toxicity. Care must be taken to minimize or prevent walnut tree roots from growing upwards into the raised beds.
Excellent soil drainage will also help reduce toxicity problems, even among sensitive plant species. In well-drained soil, toxic reactions only occur when direct contact is made between walnut roots and roots of sensitive species. In poorly drained soil direct contact between roots is not necessary to cause toxic reactions since juglone moves through the soil water. It has been suggested that plants having shallow root systems are more tolerant of juglone than deep-rooted species. Tolerance to juglone by shallow-rooted species may also be attributed to better drainage of soil water in upper soil levels.
Many sources provide long lists of plants that are sensitive to juglone and those that will tolerate it. Vegetable plants that tolerate Juglone include beans, beets, corn, onion and parsnip. Vegetable plants that are susceptible include asparagus, eggplant, pepper, potato and tomato. (Information for this column has been adapted from a fact sheet provided by the Ontario Ministry of Agriculture Food and Rural Affairs.)
The scientific term to describe the effect of juglone on other plants is allelopathy. It is a term used to describe natural interactions between plants where one plant produces a substance that affects the growth of another plant. These biochemicals influence the germination, growth, survival, and reproduction of other plants – these effects can be positive or negative.
Walnut trees are not the only plants that exhibit allelopathy. Garlic mustard, for example, is believed to excrete a chemical that interferes with the relationship between tree roots and their mycorrhizal fungi. Some varieties of rice, rye and sorghum have also been shown to provide significant weed suppression and some studies have shown that sunflowers inhibit the growth of other plants.